The present invention relates to the field of optical data storage. More specifically, in one embodiment the invention provides an improved method for making high quality, low cost optical data storage media and, in particular, high quality bump-forming media, especially erasable media.
Bump-forming, erasable optical recording media are well known. Examples of bump-forming, erasable recording media, methods of formation thereof, and methods of using such media are provided in U.S. Pat. Nos. 4,918,682 (Finegan), and 4,852,077 (Clark), as well as U.S. application Ser. No. 07/510,360 (Clark et al.), all of which are assigned to the assignee of the present invention and incorporated herein by reference for all purposes.
Write-once, CD compatible media are also known and produced by Taiyo Yuden and Mitsui Toatsu. The media are similar in that they both consist of three layers applied to a rigid, grooved substrate. Two of the layers are formed by spin coating. The third is a metal reflective layer formed by a vacuum deposition process.
Currently, the most common technique for manufacturing rigid optical media provides for the fabrication of such substrates one piece at a time on individual substrates. Layers of the media structure are formed by spin coating or vacuum deposition processes. The throughput is low because each disc is individually processed. The processing techniques, particularly spin coating, are wasteful of material. Spin coating onto a rigid substrate is commonly used for several reasons. First, it is amenable to prototype and small scale production. Secondly, the rigid substrate can be made to contain optical tracking information. This information is usually in the form of grooves, formed when the substrate was injection molded.
Often, it is necessary to form several layers of material on an optical data storage media such as the one described by Finegan, incorporated by reference above. These layers are put on in a step-wise fashion. The throughput is particularly low in such a process because for each layer, each substrate is coated individually with a spin cycle that lasts seconds to minutes. Moreover, spin coating is a wasteful process in terms of the coating solution. Most of the coating liquid is used to initially wet the substrate. The large majority of it is then spun off. Clever dispensing techniques and recovery of the spun off solution can minimize this waste. However, for a given area, spin coating continues to use substantial amounts of fluid.
Vacuum deposition of layers on an optical recording media has also been proposed. Similar throughput concerns apply to vacuum deposition processes as well. When vacuum deposition techniques are used, the media must be processed batchwise or held stationary under single coating stations.
It has also been found by the inventors herein that uniformity of the layers is impacted by piecewise processing. The substrates used in optical disc recording are typically injection molded from polycarbonate resin. As shown in prior art FIG. 1, the molding process creates a substrate 2 with a center hole 4 and a stamper ring groove 6 on the data side of the disc. The recording layers are typically coated onto the molded substrate using either vacuum deposition or spin coating processes. When spin coating recording layers, the inner diameter of the coating is affected by the diameter of the stamper ring groove and a coated/non-coated boundary condition is created. This boundary condition creates a substantial "ramp" in coating thickness such that the coating is thinner at the inner portion of the disc and thicker at the outer portion of the disc. Typically the larger the inner coating diameter the worse the wedge is for a given coating condition.
A process has been proposed for producing read-only compact discs at reduced cost as compared to standard injection molding techniques for such discs. The resulting disc structure is shown in prior art FIG. 2. An extruded polycarbonate sheet 8 is coated with a web 10 having a reflective layer 12 thereon. The web/reflective layer is connected to the polycarbonate sheet with an adhesive 14. Data marks 16 are pre-impressed into the web/reflective layer.
While prior media and methods of forming such media have met with substantial success, it is seen that an improved optical data storage media and method of forming such media is needed.